Method of and apparatus for controlling electron discharge tubes



July 13,1926. 1,592,217

M. MORRISON METHOD OF AND APPARATUS FOR CONTROLLING ELECTRON DISCHARGETUBES Filed Jan. 21, 1924 2 Sheets-Sheet 1 .TZUL /Z 2 07".-

Ju y 13 ,1926.

M. MORRISON METHOD OF AND APPARATUS FOR CONTROLLING ELECTRON DISCHARGETUBES 2 Sheets-Sheet 2 Filed Jan. 21, 1924 a f I 1720622277 W 7 PatentedJuly 13, 1926.

UNETED STATES llIONTFOR-D MORRISON,

@ATENT OFFICE.

OF CHICAGO, ILLINOIS.

Application filed January 21, 1924. Serial No. 687,516.

'llhe present invention relates to a system of connections and circuitsfor vacuum tubes of the thermionic-cathode type, such for example, X-raytubes and the like.

ldore specifically, my invention relates to the above class ofvacuum-tube devices which operate with the assistance of thermioniccathodes, and has particular reference to such devices when thedischarge produced is subject to variations in behavior due to changesin gasionization within the tube, dililerences in specific electronemissivity and instability of spontaneous electron liberation Among theobjects of my invention are; first, to provide a means for stabilizingelectron discharge of the apparatus when the source of electricalcurrent for heating the aforesaid thermionic cathodes is subject tovariation; second, to provide an improved constancy of conductivitywithin the tube when the said tube is subject to gas ionizati i third,to provide an improved constancy in the'electrical conductivity withinthe tube when the specific emissivity of the incanoescent cathodes issubject to variation; fourth, to provide an improved constancy ofconductivity within the tube when spontaneous ionization may be present,unstable or subject to variations.

Gther and further objects will be pointed out and in part obvious uponreading the specifications and inspecting the drawings, but the novelscope of my invention will be pointed out with certain particularitiesin the appended claims. 7

For a thorough understanding of my invention reference may be had to thefollowing description bearing connection with the accompanying drawingsin which I have shown three embodiments of my invention in order toillustrate more justly the scope oi my claims.

Fig. 1 is an embodiment of my invention showing my controlling apparatuslocated in the higlutension direct-current circuit or an X-ray-tubesystem employing a highvoltage rectifier.

Fig. 2 illustrates an embodiment of my invention in thehigh-tensionalternating current circuit of an X-ray-tube systememploying a high-voltage rectifier.

self, which may change for many reasons some 0t which originate withinthe cathode The art of controlling electron discharge apparatus bymechanical devices subject to discharge current through the tube, isquite old. Such apparatus has been in extensive use in connection withdischarge tubes operating with pure gas ionization, and with those inwhich only non-gas ionization is alleged to occur.

, Tie objects of controllers of the prior art or electron dischargetubes have been par ticularly directed toward the stabilization ofIll-ray tubes when the operation is supposed to talre placeindependently of gaseous ionization, and their principal objects havebeen to provide means for automatically stabilizing the thermionicemission itself without respect to any other phenomena, and have beenconceived and constructed with the idea that this one phenomenon was thesole one to be taken into consideration and their operation has beenparticularly and admittedly called for because of inherent fluctuationsin the direct sources of heating current for the thermionic cathodes, asit was believed that these heating-current variations due inherentlytoline voltage fluctuations were the principal source of operatlveerraticalness in these tubes.

However, I have discovered that there are four sources of variations inthe electrical conductivity of these tubes and that the principalditliculty does not originate from variations in filamentezvcitationvoltage but from variation in the aggregate electron emissionfrom the four. sources.

The four sources which I have discovered are:

Variations in the temperature of the c: thodc due to variations in linefluctuations heretofore referred to; variations in the specific electronemissivity of the cathode itfilament itself, and some external of it;variations in gas pressure within the tube which not only affect thegaseous ionization but also the electron emission oi the filamentitself, as well as do they even change the spect'uin of the targe due tointerference With the velocitydistribution of the electronstream; andalso variations in the spontaneous ionization of the cathode, which isthe pulling out of electrons themselves from various parts of thecathode on which the electro-static stresses are concentrated, due tolocal impurities, craters, physical-geomet rical points, sharp edges andthe like.

It will be appreciated by those familiar with the art that it iserroneous to assume that the stabilization of any one of these phenomenawill necessarily affect the stabilization of the discharge current as awhole through the tube itself. Further, if the discharge current is tobe stabilized as a whole, the aggregate electron liberation must be madeconstant as a whole, and this may be done by selecting one of thepredominating factors of the aforementioned group which may becontrolled external of the tube and by constructing device, which willadd to or subtract from the value of this selected factor in such a wayas to maintain the instantaneous aggregate electron-liberationsubstantially at a constant value.

Means and method, for doing this directly by electrical circuit systemswithout the interposition of translatin devices subject to operation bycurrents involved, are the substance of this invention.

In most of the controlling devices of the prior art some sort of relay,subject to operation by means of the discharge current through the tubehas been used to accomplish their result. In the first case on record asimple relay resembling a dArsonval metor-element, was used, in whichthe armature was actuated by the discharge current through the tube andmade to operate an osmosis regulator which might raise or lower theconductivity of the tube. In any such mechanical device constituting ameans subject to operation by the discharge current, there resides manyinherent disadvantages and difficulties due to failure of the variousmechanical parts involved to function repeatedly and continuously,identically.

In further development of the relay elementabove described, instead ofbeing ac tuated by intermittent current, it was made to vibratesynchronously with the alternations of a line voltage, and thisvibrating action in conjunction with contact-points and the like, havebeen utilized to accomplish the stabilization of discharge currentduring the process of pumping radio-telegraphic and radio-telephonicthermionic vacuum-tubes, and later X-ray tubes and the like It will beappreciated by those familiar with the art that in such vibratingsystems where the amplitude of vibration is only of the magnitude of.0015 inch, that small differences in temperature causing the expansionof parts thereof, may totally disable the device. Further, that sincethese contacts necessarily operate under currents of several ampere-s,that the local Welding of the said contacts is likely to occur and thusdisable the device. Further, that with such small clearances between thecontacts, dust, condensed moisture, and the like, are very apt toseriously interfere with the proper functioning of the stabilization asa whole. Again, these relays not only vibrate within the contactsthemselves but inherently within the trunnions due to the inherent mechanics of the system and thus rapidly wear themselves out ofadjustment. The performances imposed upon this instrument and the limitsof variation in its behavior result in its being almost impractical asan operative device and its use in the art for his reason has beenlargely abandoned.

In the present invention, I eliminate all vibrating and relay parts suchas used in the case of the vibrating relay, which is supposed to operatesynchronously with the line voltage; and further, I eliminate alldischarge-currentmechanically-operated devices, and all means subject toor responsive to any electrical condition of the current in said tubefor regulating electron emissivity of the cathode, and as contrastedwith which aforesaid devices I employ the electron discharge-currentitself, or a different current proportional thereto, to affect thedesired characteristics of the stabilization of the aggregate electronliberation.

By such method and means as constitute my invention I eliminate andobviate the use of all uncertain vibrating parts of the prior-artmechanical-devices and therefore produce a system which is aconsiderable advance over the said art.

Referring to Fl g. 1, 1 is a source of alterhating potential which bymeans of leads 2 and 3, supply primary 4 of step-up transformer 5 withelectrical energy. Primary circuit of said transformer 5 may be suppliedwith a switch 6 for practical purposes, and the primary of whichtransformer may be supplied with means 7 for adjusting the magneticinduction in the core of the said transformer in order to producedifferent voltages in the secondary 8 thereof. Leads 9 and 10 conductcurrent to the alternating current terminals 11 and 12 of a rectifier13,

which is preferably of a form fully set 1 forth in my United StatesLetters Patent No. 1,466,?1-1, issued August 28, 1923. 14: and 15 arethe direct current terminals of said rectifier. Lead 16 conductspositive electricity to the anode terminal 17 of an X-ray tube 18,preferably enclosed within a. shield 19, as set forth in my applicationSerial Number 633,712, filed April 21, 1923.

Lead 20 conducts negative current to my stabilizing apparatus, whichcurrent is thence conducted to the cathode 47 of 'X- ray tube 18.X-raytube 18 may be supplied with a current-measuring device A forpractical purposes. Alternating poten tial source 1, through lead-s 21and 22, supply transformer with energy, the circuit of which may have aswitch 24: for

pr ctical purposes. The secondary 25 of transformer 23 is supplied withthe voltage adjustment 26, and the circuit of which may contain anadjustable resistance 27, in series 'ith an adjustable inductance 28, inseries with an adjustable capacity 29.

These three circuit elements and the source of variable potential areall in series with a primary of an insulation transformer 31. i

It will be appreciated by those skilled in the art that with a source ofadjustable voltage in series with adjustable resistance, adjustableinductance and adjustable capacity, that the voltage impressed upon 30may have any root-mean-square value and phase angle with reference toalternating potential source 1, that may be desired, or requisite forthe'operation of my device. Brackets 32 are used as being symbolic ofpossible simultaneous adjustment of the four adjustable factors enclosedby them. Insulation transformer 81 supplies electrical energy for theseveral elements of my system of circuits and simultaneously electricalinsulation between high voltage lead 20 and source of electricalpotential 1.

Rectifier 13 is driven synchronously with alternating potential source1, either by direct mechanical connection or by means of synchronousmotor drive. Such connection between these elements is omitted for thesake of clearness, as this is not a novel ele ment of my invention andiswell understood by those familiar with the art.

Insulation transformer 31 has its secondary connected to leads St, whichsup-- ply exciting current for the stator of the synchronous motor 35',driving a commutator 86. The armature and several mechanical parts ofthe said synchronous motor is not brought into View for thesake ofclearness. ply filament 47 of X-ray tube 1'8 with electrical heatingenergy through concentric conductors 36. The circuit from the secondarywhich supplies the aforesaid heating current to the filament 47, hasconnected in series with it the primary 37 of a current transformer 38,and the said .concentric leads 36 have shunted across them an inductiveresistance 39 at the middle point of which is connected a lead 40, whichsupplies conductivity to brush 41 of commutator 36. The two halfelements of said resistance, formed by connecting lead l0 to resistor39, are shunted by condensers l2 and 43.

Secondary leads 34, also sup- Lead 20 is conducted to brush 4A: ofcommutat-or 36 through two equal inductances 45 and 46. Gommutator 36 isshunted by condenser 48; inductance l6 and commutator 36, are jointlyshunted by condenser 49. Inductances 45 and a6, and condensers 48 and49, form a filter circuit which may be adjusted to selectively filterout, of the circuit including the commutator 36, such higher harmoniccomponents of the X-ray tube current as may be desirable oradvantageous.

The function of the middle connection to resistor 39 is to allow thenegative current arriving from lead 20 to divide equally be tween thetwo concentric conductors 36, which in conjunction with the shuntcondensers 42 and 43 provides a balance of conditions between thefilament heating cur rent circuit and the discharge current circuit ofthe X-ray tube, such that difficulties from high frequency phenomena areminimized.

Current transformer 38 is provided with a secondary 50, across which isshunted brushes 51 and 52 of commutator 36. The operation of thisapparatus is as follows:

Source of alternating current 1 is made to function; switch 24 isclosed. Transformer 23 is supplied with electrical energy and insulationtransformer 33- is in turn supplied with such voltage at such phaseangle, by means of adjustment of voltage 25, resistance at 27,inductance at 28 and capacity at 29, as will give the desired phaserelation between the current in the primary 37 of current transformer38, and that of its secondary 50, as will be more fully hereinafterappreciated.

Transformer 31 receiving energy, imparts excitation to stator 35, whichstarts andop crates commutator 36 synchronously with alternating currentsource 1, and with the proper phase relation and polarity with respectto the currents involved in its functioning. Further, current from,secondary 3S flows through primary 3'? to filament 47, giving the saidfilament theproper excitation to emit the desired number of electrons.

The reactance of transformer 38 has the effect of giving a slightdifferent excitation to filament 47 in the absence of current throughsecondary 50, but this is not materially undesirable or detrimental tothe operation of my device. Under the condition of operatioi'i, fareriplained, resistor 39 and condensers 42 and 43 take a small amount ofcurrent from secondary 33, but this plays no essential role in theoperation of the apparatus.

Pectiiier 13 being connected mechanically or synchronously toalternating current source 1, is in proper rotation, phase angle andpolarity, simultaneously with the proper functioning of alternatingcurrent source 1,

so that switch 6 may be now closed, adjustment '7 manipulated so thatsecondary 8 of transformer 5 has its proper operating voltage, uponwhich status of affairs X-ray tube 18 is supplied with mainelectrode-discharge current.

The current from lead 20, being conducted to brush 4%, is directedthrough secondary 50 before it arrives at brush 41, after which it isdirected to cathode 17 through condensers 42 and 43 and resistor 39.Resistor 39 is illustrated as being wound inductively, as in theoperation of this inductive resistance, in conjunction with the shuntedcondensers, the inductive effect in the said inductive resistance is notapparent in the average current passed under the conditions set forth inthis case, namely; that of the passage of unidirectional current.

The function of the commutator 36, is to redirect the pulsatingrectified currents of the main electrodes of the X-ray tube 18 so thatthey re-torm an alternating current, in secondary 50 which was theiroriginal form in secondary 8.

The utilization of this discharge. current in its alternating forminstead or its direct current form is for the purpose of eon'ibining itinductively in secondary 5'0 with the sinusoidal current in primary 3?,will now be explained. The current through primary 37, is of the commonsinusoidal form. The current in 50 is of the magnitude of a fewmilliamperes, and the current of primary 37 is of the magnitude ofseveral ampereS. The current transformer 38 and its ratio oftransformation is called for and fixed such that the effects of thesetwo currents on the magnetic induction on the core of transformer 38 arecomparable in maguitude and hence these two currents may be made tocombine in effect much as if they were of the same order oi currentthemselves.

The combined etlect of these two currents under the condition specifiedis such that the filament excitation current assumes a compromised formand it will be appreciated that the greater the discharge currentthrough the tube the greater amplitude is given to curve the current ina secondary 50, hence the greater amount is subtracted from the heatingcurrent in primary 37, leaving less of it for filament excitation, whichmeans lower thermionic discharge currents through the tube.

It will be appreciated by those familiar with the art relating todischarge tubes that the thermionic discharge current is not directlyproportional to the filament excitation current, but after theexcitation current has risen considerably the thermionic current risesmuch more rapidly, and hence at high filament excitation currents thethermionic discharge current is very sensitive to small differences inthe said excitation current. This means, with relatively small amountsof current taken from or added to the heating current, after it has beenallowed to rise to a considerable value, that in the present inventionthe thermionic discharge current can be made to vary considerable evenby small difierences of discharged current. This means that it any oneof the aforementioned variations in conductivity of the X-ray tube comesinto play, the slight increase or decrease in the said conductivitytends to produce a. large difference in ther mionic emission, thusgiving a very sensitive stabilizing efiect to the conductivity of theX-ray tube.

In Fig. 2 is shown a system of connections attaining the same ultimateresults as the system in Fig. 1, the only difference being that in Fig.1 the control system is in the higlrvoltage pulsating-current circuit ofthe X-ray tube and, therefore, the commutator is required to turn themain electrooe discharge current back into alternating current, in orderthat it may operate in proper combination with the alternating excitingcun rent of the X-ray tube filament.

In Fig. 2 the control system is located in a certain alternating-currentportion of the high-voltage circuit, which portion is so in sulated thatit may be grounded and the controlling-system commutator is notrequired. Otherwise the operation is substantially the same.

In Fig. 2, all parts common to both figures have been given similarnumerals, and this includes all numerals from 1 to 34: inclusive, and to49 inclusive, and with this understanding the repetition of theidentical descriptive matter is eliminated.

It will be appreciated that the main electrode current of the X-ray tube18, as it flows through secondary 8, is alternating in character, as ithas not been commutated by rectifier 13 at this time. Inductances l5 and46, and condensers 48 and 49 will be ob served to constitute the samefilter as indi cated by these numerals in Fig. 1.

Secondary 8 has the center of its winding connected to this filter andshunted by a three-electrode air gap 53, one electrode of which isconnected to the center of the highturn winding 50 of the currenttransformer 38, which is common to both figures, and said transformer issupplied with a high-frequency and static-potential-shield 54, common intransformer art, which shield is grounded. In addition, the high-voltagefilter-circuit is supplied with resistors and 56, which over and abovethe action of the filter proper not only assists in spark-over at gap 53in the case of surges, and the like;

but also limits the short circuit current in this filter circuitduring'sparh-over when energy is being transmitted to the circuitthrough primary 3?, which is the case under operating conditions.

It will be appreciated by the art that with onl" t such parts of Fig.52- i l by similarity to Fig, 1 ant the description of F operation ofthis set-up same as that of Fig. 1.

As appreciated by those sl i led in the the current in primary 4 oftransformer very similar and closely proportioned to the current insecondary 8 of the said transformer, when the exciting current componentof the transformer supply current does not form a substantial factor inthe total primary current. Therefore in some cases I may desire to closethe switch 57, open the switch 58, and open switch 5'9, which throwsinto circuit in place of transformer a transformer having a winding 61similar to that of 38 indicated by numeral 37, and a low-turn winding 62which i called for to be different than 50 by reason of the ratio oftransformation in transformer it will be observed that this is merely atranster of stabilizing circuits from the secondary of the high-voltagetransformer to the primary circuit, and the theory of operationissubstantially the same as that already described, in the case of a lowexciting current transformer being used at 5.

It will be appreciated by those skilled in the art that the inventiondescribed and the embodiments illustrated, comprise a system of controlwhich has no parts nor means responsive to electrical conditions ofcurrent transmitted by the tube for stabilizing the tube operation, butall such parts and means have been eliminated; and the stabilization ofthe tube conductivity, with particular consideration of the aggregateelectron liboration, is caused by the electrical condition oi thecurrent itself without other means.

W hat I claim is 1. The combination of a thermionic Xray tube havingconductivity subject .to variations in the temperature of its cathode,va riations in the specific emmissivity of said cathode, variations inthe gas pressure within the envelope of said cathode, variations in thespontaneous liberation at said cathode, a high voltage rectifier forcommutating voltage for the X-ray tube mainelectrodes, anindependentsource of alternating heating-current for the cathode of saidtube, and means for converting the rectified tube current, in anauxiliary circuit associated with the said heating current back intoalternating current, whereby the interaction between the said heatingcurrent and the said converted tube current causes said tube to tho e -loperate according; to a desired characteristic.

The combination oi? thermionic X-ray tube having conductivity subject tovariations in the temperature of its cathode, variations in the specificemmissivity of said cathode, variations in the gas pressure with in theenvelope of said cathode, variations in the spontaneous liberation atsaid cathode, a high vol age rectifier ior commutating voltage for theTLray tube main electrodes, an independent source of alternatingheating-current tor the cathode of the said tube, and means forassociating the current of the main electrodes in alternating currentform with the heating current or the said elect ode, whereby theinteraction between said heating current and the said current or" themain elec trodes in the said form causes the tube to operate accordingto the desired characteristic;

3. The combination oi a thermionic X-ray tube having conductivitysubject to vauations in the temperature of its cathode, variations inthe specific emissivity of said cathode, variations in the gas pressurewith in the envelope of said cathode, variations in the spontaneousliberation at said cathode, a high voltage rectilier for conimutatingvoltage for the X-ray tube main-electrodes, an independent source ofalternating heating-current for the cathode of the said tube, means forassociating the current of the mainelectrodes with the heating currentof the said cathode, and means toradjusting the phase relation betweenthe current of the main-electrodes and the said heating current, wherebythe conductivity of the tube is caused to assume the desiredcharacteristic.

4:. The coml ination of a thermionic X-ray tube having conductivitysubject to variations in the temperature of its cathode, variations inthe specific emissivity of said cathode, variations in the gas pressurewithin the envelope of said cathode, a source of high potential currentfor the X-ray tube 1Ilfll]18lCtlOClS, an independent source ofalternating heating-current tor the cathode of the said tube, means forassociating the cur rent of the main electrodes with the heating currentof the said cathode, and means for adjusting the phase relation betweenthe current of the main-electrodes and the said heating current, wherebythe conductivity of the tube is caused to assume the desiredclulractcristic.

5. The combination of a thermionic X-ray tube having conductivitysubject to variations in the temperature oi its cathode, variations inthe specific emissivity of said cathode, variations in the gas pressurewithin the envelope of said cathode, variations in the spontaneousliberationat said cathode,

a high voltage I. tiller for commutating voltage for the X-ray tube mainelectrodes, an independent source of alternating heating current for thecathede of said tube, means for associating the current of the mainelectrodes with the heating current of the said cathode, and means foradjusting the specific value of the said association,

whereby the range of stable main-electrode current may be fixed to adesired meanvalue.

In Witness whereof, I have hereunto set my hand this seventeenth day ofJanuary,

MONTFORD MORRISON.

